Heat-shrinkable products have been used increasingly in recent years for a variety of purposes, including the sealing of cable or wire joints or splices and, more generally, the application of an insulating sleeve in electrical and like applications. A heat-shrinkable tubular section, hereinafter referred to as a heat-shrinkable sleeve, can be placed over a splice in a cable and subjected to heat from a radiant heating source, a burner, a contact heater or the like. The heat causes the shrinkable material to contract and form a tight seal around the joint which can be electrically insulating and hermetic.
Heat-shrinkable products can also be provided in the form of a tape which can be wrapped around electrical or other joints and which can be heated to the shrinking temperature to increase the tension of the type around the body to which it is applied and to effect fusion or adhesion of the inner surface of the heat-shrinkable material to the material to which it is applied.
In general with heat-shrinkable materials, the temperature development in the product is of considerable importance since the heat-shrinking operation requires a thermal softening and a subsequent hardening of the product to achieve the seal and the bonding to the body to which the product is applied. The temperature to which the material is brought is of special importance with heat-shrinkable materials which have an adhesive layer.
With temperatures which are too low, the adhesive layer may not melt and thus may not provide a satisfactory bond because there is insufficient adhesion of the shrunk material to the body to which it is applied.
With an excessively high temperature, the adhesive layer can be damaged or the heat-shrinkable material to which the adhesive layer is applied can be injured.
To avoid these drawbacks, it has been proposed to include thermally-sensitive coloring agents in or on the shrink products, these agents or substances providing certain color changes or color developments when appropriate temperatures are reached (see U.S. Pat. No. 3,816,335, U.S. Pat. No. 4,105,583, U.K. patent 1,511,053 and U.K. patent 2,038,478). Coatings of these thermally-sensitive coloring agents, however, provide no additional function and have not always been fully satisfactory.
The products made with these materials, for example, can be susceptible to insects, microorganisms and rodents and may suffer deterioration from such pests.
It is also known to provide a heat-shrinkable article of the aforedescribed type with at least one layer which, upon reaching the shrinking temperature, becomes transparent. The heat-shrinkable material or object can thus be provided of a particular color with a coloring agent, pigment or dyestuff which imparts a hue or color to the shrinkable material which is different from the color of the object to which the material is applied.
The heat-shrinkable article may itself comprise two cross-linked superposed layers of which only the inner layer is colored while the outer layer is not colored or is only weakly colored. In this system, the outer layer is not initially transparent, but rather is opaque to a greater or lesser extent so that the inner layer cannot be seen and its color is not ascertainable.
Only upon heating of the outer layer and the attainment of the desired temperature, which should lie as close as possible to the optimum shrinking temperature, does the outer layer become suddenly transparent so that the inner layer will become visible (see German patent document DE-OS 33 41 617).